Equilibria of copper(II) acetate with acetylacetone in acetic acid and methanol solutions from optical and electron spin resonance measurements

Abstract

The equilibria of the reaction of copper acetate with acetylacetone in acetic acid and methanol solutions have been studied using e.s.r. and spectrophotometric measurements. In dry methanol the dimer dissociation constant K_D=[CuAc₂]²/[Cu₂Ac₄]≈ 6 × 10^–3 M and the monomer dissociation constant K_M=[CuAc⁺][Ac^–]/[CuAc₂]≈ 3 × 10^–4 M (Ac^–= acetate ion). Small additions of acetylacetone almost completely chelate with copper forming the mixed-ligand monomeric complex, CuAc(acac), and the bisacetylacetonate complex, Cu(acac)₂. The mixed-ligand complex is slightly more stable than expected statistically. In acetic acid, however, only the bisacetylacetonate complex is formed according to the equilibrium Cu₂Ac₄+ 4Hacac 2Cu(acac)₂+ 4HAc. The equilibrium constant, K=[Cu(acac)₂]²/[Cu₂Ac₄], increases as log K= 2.6 + 3.72 log [Hacac].